The present invention relates to a magnetic disk drive and, more particularly, to a magnetic disk drive having a circuit for controllably driving a stepping motor which moves a magnetic head.
A magnetic drive disk such as a floppy disk drive has customarily been provided with a stepping motor or similar drive source for driving a magnetic head. A stepping motor for this kind of application is driven by current values which are representative of desired driving conditions. With a conventional floppy disk drive, only a single combination of current values is available for driving a stepping motor as disclosed in U.S. Pat. Nos. 4,788,608 and 4,831,471. In the conventional floppy disk drive, a stepping motor exerts a static torque having a constant gradient. Hence, if the absolute value of the static torque is increased in order to increase the moving speed of the motor, the specific oscillation frequency of the motor in the energized state and, therefore, the frequency of oscillation which remains after the deenergization of the motor will be increasred. In this condition, assuming that the stepping motor is controlled on a sector servo basis which is well known in the art, then the servo cannot act on the motor immediately due to the oscillation of the motor. Especially, when the oscillation frequency approaches the sampling frequency of a servo motor, the servo will practically fail to work because the oscillation is apparently stopped despite that it is under way.